In recent years, as a light source of an image display device such as a projector, there has been developed, for example, an optical device that includes: a light-emitting element; a light guide on which light (excitation light) from the light-emitting element is incident; a carrier generation layer that is provided in the light guide and in which carriers are generated by the light from the light guide; a plasmon excitation layer that excites a plasmon, which is stacked on the upper side of the carrier generation layer and has a plasma frequency higher than a frequency of light generated when the carrier generation layer is excited by the light from the light-emitting element; and an exit layer stacked on the plasmon excitation layer, which converts light incident from the plasmon excitation layer into light having a predetermined exit angle and from which the light having the predetermined exit angle exits (Patent Document 1).
Such an optical device emits light based on the principle to be described below. First, the carrier generation layer absorbs excitation light emitted from the light-emitting element, whereby carriers are generated in the carrier generation layer. These carriers couple to free electrons in the plasmon excitation layer and thus excite a surface plasmon. Then, the excited surface plasmon is emitted as light.